The halogenation and attempted epoxidation of 3a,8a-dihydrofuro[2,3-b]benzofuran and aflatoxin B1

Abstract

Bromine adds across the enol ether moiety of 3a,8a-dihydrofuro[2,3-b]benzofuran and aflatoxin B₁ to produce exclusively trans-dibromides, whereas addition of chlorine results in a mixture of trans- and cis-dichlorides. The resultant dibromides undergo nucleophilic substitution at C-2 or C-8, respectively, with retention of configuration, but the analogous substitution on the dichlorides is not so stereospecific. Peroxoacid oxidation of 3a,8a-dihydrofuro[2,3-b]benzofuran results in the addition of the peroxoacid across the terminal furan double bond via an epoxide or a resonance-stabilised carbonium ion intermediate to give cis- and trans-2-(3-chlorobenzoyloxy)-3-hydroxy-2,3,3a,8a-tetrahydrofuro[2,3-b]benzofuran and trans-8-(3-chlorobenzoyloxy)-9-hydroxy-8,9-dihydroaflatoxin B₁. The cis-2-(3-chlorobenzoyloxy)-3-hydroxy-2,3,3a,8a-tetrahydrofuro[2,3-b]benzofuran ester rearranges in the reaction conditions to give trans-3-(3-chlorobenzoyloxy)-2-hydroxy-2,3,3a,8a-tetrahydrofuro[2,3-b]benzofuran. When ethanol is added to the epoxidising system it acts as a competing nucleophile to give cis- and trans- hydroxy-acetals.